1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method.
2. Description of the Related Art
As exemplary image forming apparatuses for forming an image by performing a print scan a plurality of times upon the same image area on a print medium, ink-jet printers are known.
Ink-jet printers print an image on a print medium by repeatedly performing an operation of causing a printhead to eject ink droplets onto the print medium while reciprocally moving in a main scanning direction and an operation of conveying the print medium in a sub-scanning direction. In such an ink-jet printer, the variations in the orientation and size of an ink droplet and the position at which an ink droplet lands occur at the time of printing due to errors caused by physical factors such as the characteristic differences among nozzles and the inaccuracies of a sheet conveying distance and a printhead moving distance. These variations appear as density unevenness or stripes on an image printed by a single print scan, and thus degrade image quality.
As a method of preventing the occurrence of such density unevenness and stripes, a multi-pass printing method is known. The multi-pass printing method is a method of performing image processing and printing control in combination, and can achieve rapid image formation while suppressing degradation in image quality due to density unevenness or stripes.
The multi-pass printing method will be described in detail below with reference to FIG. 12.
Referring to FIG. 12, a printhead 5101 is composed of nozzles 5102. For simplification of explanation, it is assumed that the printhead 5101 is composed of eight nozzles 5102. From the nozzles 5102, ink droplets 5103 are ejected. In general, in order to complete printing of a print area on a predetermined print medium by a single print scan, it is desirable that the same amount of ink be ejected from the nozzles 5102 in the same direction as illustrated in FIG. 12.
However, as described previously, if printing of an image is performed by a single print scan, the size and orientation of an ink droplet ejected from a nozzle vary from nozzle to nozzle due to an error caused by a physical factor at the time of printing. As a result, a blank portion periodically appears or an excessive number of dots overlap one another in a head main scanning direction on a print medium. A group of dots that land in this state is perceived as density unevenness in a nozzle array direction on a print medium. If there is a misalignment between print areas printed in print scans, the boundary between these print areas is perceived as a stripe.
In the multi-pass printing method, as illustrated in FIG. 13, a printhead 5201 performs a print scan a plurality of times (three times in this case). Referring to FIG. 13, printing of each print area including four pixels (half the length in the vertical direction in which eight pixels are arranged) is completed by two print scans. In this case, eight nozzles 5202 included in the printhead 5201 are divided into a group of four upper nozzles (upper nozzle group) and a group of four lower nozzles (lower nozzle group). A dot printed by each nozzle in a single print scan corresponds to data obtained by thinning out image data to about half in accordance with a predetermined image data arrangement. In a second print scan, dots corresponding to about the remaining half of the image data are embedded into an image formed in the first print scan, so that printing of a four-pixel unit area is completed.
In the multi-pass printing method, for example, a two-pass printing method, a first print scan and a second print scan complement each other in accordance with a predetermined image data arrangement. As the predetermined image data arrangement (thinning-out mask pattern), an arrangement illustrated in FIG. 14 in which pixels are vertically and horizontally staggered one by one is generally used. Accordingly, printing of a print unit area (a four-pixel unit area in this case) is completed by a first print scan for printing a staggered pattern and a second print scan for printing an inverse-staggered pattern. FIG. 14 illustrates a process of completing printing of the same area using the staggered and inverse-staggered patterns. That is, as illustrated in the upper part of FIG. 14, in a first print scan, printing of the staggered pattern (black circles) is performed in a predetermined region on a print medium using the four lower nozzles. Subsequently, as illustrated in the middle part of FIG. 14, in a second print scan, a sheet is fed by four pixels, and printing of the inverse-staggered pattern (white circles) is performed in the area on the print medium using all of the eight nozzles. Subsequently, as illustrated in the lower part of FIG. 14, in a third print scan, the sheet is fed by four pixels, and printing of the staggered pattern is performed again in the region on the print medium using the four upper nozzles.
Even if a multi-head with variations like those illustrated in FIG. 13 is used, the multi-pass printing method can reduce the influence of the variations on a print area by half. Even if there is a misalignment between print areas printed in print scans, the multi-pass printing method can reduce the influence of the misalignment by half. As a result, density unevenness is reduced on a formed image. An exemplary case in which printing of a unit area is completed by two print scans has been described. If the number of print scans is increased, the influence of the above-described variations or the above-described misalignment can be further minimized. Accordingly, the density unevenness can be reduced in proportion to the number of print scans. Conversely, the time for printing is increased in proportion to the number of print scans.
If the number of print scans is reduced so as to perform high-speed printing, it is difficult to average the variations in the ink droplet or the misalignment between passes, and the density unevenness is therefore more pronounced than that in a case where the number of print scans is not reduced. Accordingly, in order to improve image quality in high-speed printing in which a small number of print scans are performed, a dot arrangement is required which has a characteristic highly resistant to the variations in the ink droplet or the misalignment between passes (a characteristic highly resistant to reduction in image quality).
A technique for creating from image data print data used for each print scan by performing thinning with a random thinning-out pattern that uses random numbers or the like is known. For example, it is assumed that printing is performed by two print scans using the above-described technique. In a first print scan, thinning is performed with a random thinning-out pattern that uses random numbers or the like, and in a second print scan, thinning is performed with the inverse thinning-out pattern of the random thinning-out pattern, so that each pieces of print data is created. In this case, there is no regularity in a dot arrangement, and image quality is therefore improved as compared with printing in the related art in which two print scans are performed. As described previously, however, the variations in the ink droplet and the misalignment between print scans occur at the time of printing. In the above-described technique, since the complementary relationship between print scans is formed by performing thinning using a mask pattern in each of the print scans, the variations in the ink droplet and the misalignment between print scans lead to the overlapping of dots and the periodic appearance of a blank portion which are easily perceived as density unevenness. In particular, if dot patterns interfere with each other due to the misalignment between print scans, density unevenness and stripes appear as an undesirable pattern after scanning.
It is therefore required to prevent any dot patterns created in print scans from interfering with each other in a case where the misalignment between the print scans occurs. However, it is difficult to create a mask pattern capable of preventing the interference between dot patterns which can be used for any input image.
As a method of overcoming the above-described difficulties, a method of dividing each pixel value which is multi-valued image data into pieces of multi-valued image data that are individually used for print scans, quantizing these pieces of multi-valued image data, and generating print scan images between which there is an incomplete complementary relationship using these pieces of multi-valued image data is known. This method can reduce the influence of the variations in the ink droplet or the misalignment between passes on image density, and improve image quality.
However, if print scan images are generated using the above-described method, quantization is performed in each of the print scans. Thus, in this method, the relationship between dot arrangements obtained by the print scans is not taken into account. As a result, in dot patterns generated in the print scans, dot sparse/dense portions may appear. These portions are perceived as density unevenness on a printed image, and therefore become the cause of the reduction in image quality. In particular, in a low-density portion on the printed image, dots obtained in passes are close to each other and a blank portion is present. Thus, dot sparse/dense portions are apt to be conspicuous. FIG. 15 is a diagram illustrating an arrangement of dots generated from an input image of uniform density (low density) using the above-described method. As is apparent from FIG. 15, dots are nonuniformly arranged, and some of these dots overlap each other. Accordingly, in order to improve image quality, it is required to take a dot arrangement in a low-density portion into account.